Dice bread into half-inch pieces, drizzle with olive oil and toss in salt. Spread bread on baking sheet and bake for 10 minutes at 250°F (or until just crunchy). Combine cucumber, zucchini, squash, bell pepper, red onion, tomatoes, basil, and bun pieces in a large bowl. Drizzle with vinaigrette. Season with salt and pepper and toss gently.